How Diesel Locomotives Work


The hybrid diesel locomotive is an incredible display of power and ingenuity. It combines some great mechanical technology, including a huge, 12-cylinder, two-stroke diesel engine, with some heavy duty electric motors and generators, throwing in a little bit of computer technology for good measure.

This 270,000-pound (122,470-kg) locomotive is designed to tow passenger-train cars at speeds of up to 110 miles per hour (177 kph). The diesel engine makes 3,200 horsepower, and the generator can turn this into almost 4,700 amps of electrical current. The four drive motors use this electricity to generate over 64,000 pounds of thrust. There is a completely separate V-12 engine and generator to provide electrical power for the rest of the train. This generator is called the head-end power unit. The one on this train can make over 560 kilowatts (kW) of electrical power.

This combination of diesel engine and electric generators and motors makes the locomo­tive a hybrid vehicle. In this article, we'll start by learning why locomotives are built this way and why they have steel wheels. Then we'll take a look at the layout and key components.


Why Hybrid? Why Diesel?

The 3,200-horsepower engine that drives the main generator
The 3,200-horsepower engine that drives the main generator

The main reason why diesel locomotives are hybrid is because this eliminates the need for a mechanical transmission, as found in cars. Let's start by understanding why cars have transmissions.

Your car needs a transmission because of the physics of the gasoline engine. First, any engine has a redline -- a maximum rpm (revolutions per minute) value above which the engine cannot go without exploding. Second, if you have read How Horsepower Works, then you know that engines have a narrow rpm range where horsepower and torque are at their maximum. For example, an engine might produce its maximum horsepower between 5,200 and 5,500 rpm. The transmission allows the gear ratio between the engine and the drive wheels to change as the car speeds up and slows down. You shift gears so that the engine can stay below the redline and near the rpm band of its best performance (maximum power).

The five- or six-speed transmission on most cars allows them to go 110 mph (177 kph) or faster with an engine-speed range of 500 to 6,000 rpm. The engine on our diesel locomotive has a much smaller speed range. Its idle speed is around 269 rpm, and its maximum speed is only 904 rpm. With a speed range like this, a locomotive would need 20 or 30 gears to make it up to 110 mph (177 kph).

A gearbox like this would be huge (it would have to handle 3,200 horsepower), complicated and inefficient. It would also have to provide power to four sets of wheels, which would add to the complexity.

By going with a hybrid setup, the main diesel engine can run at a constant speed, turning an electrical generator. The generator sends electrical power to a traction motor at each axle, which powers the wheels. The traction motors can produce adequate torque at any speed, from a full stop to 110 mph (177 kph), without needing to change gears.

Why Diesel?

Diesel engines are more efficient than gasoline engines. A huge locomotive like this uses an average of 1.5 gallons of diesel per mile (352 L per 100 km) when towing about five passenger cars. Locomotives towing hundreds of fully loaded freight cars use many times more fuel that this, so even a five or 10 percent decrease in efficiency would quickly add up to a significant increase in fuel costs.

Steel Wheels

Ever wonder why trains have steel wheels, rather than tires like a car? It's to reduce rolling friction. When your car is driving on the freeway, something like 25 percent of the engine's power is being used to push the tires down the road. Tires bend and deform a lot as they roll, which uses a lot of energy.

The amount of energy used by the tires is proportional to the weight that is on them. Since a car is relatively light, this amount of energy is acceptable (you can buy low rolling-resistance tires for your car if you want to save a little gas).

Since a train weighs thousands of times more than a car, the rolling resistance is a huge factor in determining how much force it takes to pull the train. The steel wheels on the train ride on a tiny contact patch -- the contact area between each wheel and the track is about the size of a dime.

By using steel wheels on a steel track, the amount of deformation is minimized, which reduces the rolling resistance. In fact, a train is about the most efficient way to move heavy goods.

The downside of using steel wheels is that they don't have much traction. In the next section, we'll discuss the interesting solution to this problem.


Traction when going around turns is not an issue because train wheels have flanges that keep them on the track. But traction when braking and accelerating is an issue.

This locomotive can generate 64,000 pounds of thrust. But in order for it to use this thrust effectively, the eight wheels on the locomotive have to be able to apply this thrust to the track without slipping. The locomotive uses a neat trick to increase the traction.

In front of each wheel is a nozzle that uses compressed air to spray sand, which is stored in two tanks on the locomotive. The sand dramatically increases the traction of the drive wheels. The train has an electronic traction-control system that automatically starts the sand sprayers when the wheels slip or when the engineer makes an emergency stop. The system can also reduce the power of any traction motor whose wheels are slipping.

Now let's check out the layout of the locomotive.

The Layout: Main Engine and Generator

Nearly every inch of the 54-ft (16.2-m) locomotive is tightly packed with equipment.


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Mouse over the part labels to see where each is located on the diesel engine.


Main Engine and Generator

The giant two-stroke, turbocharged V-12 and electrical generator provide the huge amount of power needed to pull heavy loads at high speeds. The engine alone weighs over 30,000 pounds (13,608 kg), and the generator weighs 17,700 pounds (8,029 kg). We'll talk more about the engine and generator later.

The Layout: Cab and Trucks

The view from the cab of the locomotive
The view from the cab of the locomotive

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Mouse over the part labels to see where each is located on the diesel engine.



The cab of the locomotive rides on its own suspension system, which helps isolate the engineer from bumps. The seats have a suspension system as well.

Inside the cab there are two seats: one for the engineer and one for the fireman. The engineer has easy access to all of the locomotive's controls; the fireman has just a radio and a brake control. Also inside the car, right in the nose of the locomotive, is a toilet.


The trucks are the complete assembly of two axles with wheels, traction motors, gearing, suspension and brakes. We'll discuss these components later.

The Layout: Power, Fuel and Batteries

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Mouse over the part labels to see where each is located on the diesel engine.


Head-end Power Unit

The head-end power unit consists of another big diesel engine, this time a four-stroke, twin-turbocharged Caterpillar V-12. The engine itself is more powerful than the engine in almost any semi-truck. It drives a generator that provides 480-volt, 3-phase AC power for the rest of the train. This engine and generator provide over 560 kW of electrical power to the rest of the train, to be used by the electric air conditioners, lights and kitchen facilities. By using a completely separate engine and generator for these systems, the train can keep the passengers comfortable even if the main engine fails. It also decreases the load on the main engine.

Fuel Tank

This huge tank in the underbelly of the locomotive holds 2,200 gallons (8,328 L) of diesel fuel. The fuel tank is compartmentalized, so if any compartment is damaged or starts to leak, pumps can remove the fuel from that compartment.


The locomotive operates on a nominal 64-volt electrical system. The locomotive has eight 8-volt batteries, each weighing over 300 pounds (136 kg). These batteries provide the power needed to start the engine (it has a huge starter motor), as well as to run the electronics in the locomotive. Once the main engine is running, an alternator supplies power to the electronics and the batteries.

Let's take a more detailed look at some of the main systems on the locomotive.

The Engine and Generator

The main engine in this locomotive is a General Motors EMD 710 series engine. The "710" means that each cylinder in this turbocharged, two-stroke, diesel V-12 has a displacement of 710 cubic inches (11.6 L). That's more than double the size of most of the biggest gasoline V-8 car engines -- and we're only talking about one of the 12 cylinders in this 3,200-hp engine.

So why two-stroke? Even though this engine is huge, if it operated on the four-stroke diesel cycle, like most smaller diesel engines do, it would only make about half the power. This is because with the two-stroke cycle, there are twice as many combustion events (which produce the power) per revolution. It turns out that the diesel two-stoke engine is really much more elegant and efficient than the two-stroke gasoline engine. See How Diesel Two-Stroke Engines Work for more details.

You might be thinking, if this engine is about 24 times the size of a big V-8 car engine, and uses a two-stroke instead of a four-stroke cycle, why does it only make about 10 times the power? The reason is that this engine is designed to produce 3,200 hp continuously, and it lasts for decades. If you continuously ran the engine in your car at full power, you'd be lucky if it lasted a week.

Here are some of the specifications of this engine:

  • Number of cylinders: 12
  • Compression ratio: 16:1
  • Displacement per cylinder: 11.6 L (710 in3)
  • Cylinder bore: 230 mm (9.2 inches)
  • Cylinder stroke: 279 mm (11.1 inches)
  • Full speed: 904 rpm
  • Normal idle speed: 269 rpm

This giant engine is hooked up to an equally impressive generator. It is about 6 feet (1.8 m) in diameter and weights about 17,700 pounds (8,029 kg). At peak power, this generator makes enough electricity to power a neighborhood of about 1,000 houses!

So where does all this power go? It goes into four, massive electric motors located in the trucks.

The Trucks: Propulsion & Suspension

The trucks are the heaviest things on the train -- each one weighs 37,000 pounds (16,783 kg). The trucks do several jobs. They support the weight of the locomotive. They provide the propulsion, the suspensions and the braking. As you can imagine, they are tremendous structures.


The traction motors provide propulsion power to the wheels. There is one on each axle. Each motor drives a small gear, which meshes with a larger gear on the axle shaft. This provides the gear reduction that allows the motor to drive the train at speeds of up to 110 mph.

Two of the traction motors removed from a truck
Two of the traction motors removed from a truck

Each motor weighs 6,000 pounds (2,722 kg) and can draw up to 1,170 amps of electrical current.


The trucks also provide the suspension for the locomotive. The weight of the locomotive rests on a big, round bearing, which allows the trucks to pivot so the train can make a turn. Below the pivot is a huge leaf spring that rests on a platform. The platform is suspended by four, giant metal links, which connect to the truck assembly. These links allow the locomotive to swing from side to side.

The weight of the locomotive rests on the leaf springs, which compress when it passes over a bump. This isolates the body of the locomotive from the bump. The links allow the trucks to move from side to side with fluctuations in the track. The track is not perfectly straight, and at high speeds, the small variations in the track would make for a rough ride if the trucks could not swing laterally. The system also keeps the amount of weight on each rail relatively equal, reducing wear on the tracks and wheels.

The Trucks: Braking

The brakes are similar to drum brakes on a car.
The brakes are similar to drum brakes on a car.

Braking is provided by a mechanism that is similar to a car drum brake. An air-powered piston pushes a pad against the outer surface of the train wheel.

In conjunction with the mechanical brakes, the locomotive has dynamic braking. In this mode, each of the four traction motors acts like a generator, using the wheels of the train to apply torque to the motors and generate electrical current. The torque that the wheels apply to turn the motors slows the train down (instead of the motors turning the wheels, the wheels turn the motors). The current generated (up to 760 amps) is routed into a giant resistive mesh that turns that current into heat. A cooling fan sucks air through the mesh and blows it out the top of the locomotive -- effectively the world's most powerful hair dryer.

On the rear truck there is also a hand brake -- yes, even trains need hand brakes. Since the brakes are air powered, they can only function while the compressor is running. If the train has been shut down for a while, there will be no air pressure to keep the brakes engaged. Without a hand brake and the failsafe of an air pressure reservoir, even a slight slope would be enough to get the train rolling because of its immense weight and the very low rolling friction between the wheels and the track.

The hand brake is a crank that pulls a chain. It takes many turns of the crank to tighten the chain. The chain pulls the piston out to apply the brakes.

Driving a Locomotive

You don't just hop in the cab, turn the key and drive away in a diesel locomotive. Starting a train is a little more complicated than starting your car.

The engineer climbs an 8-foot (2.4-m) ladder and enters a corridor behind the cab. He or she engages a knife switch (like the ones in old Frankenstein movies) that connects the batteries to the starter circuit. Then the engineer flips about a hundred switches on a circuit-breaker panel, providing power to everything from the lights to the fuel pump.

Next, the engineer walks down a corridor into the engine room. He turns and holds a switch there, which primes the fuel system, making sure that all of the air is out of the system. He then turns the switch the other way and the starter motor engages. The engine cranks over and starts running.

Next, he goes up to the cab to monitor the gauges and set the brakes once the compressor has pressurized the brake system. He can then head to the back of the train to release the hand brake.

Finally he can head back up to the cab and take over control from there. Once he has permission from the conductor of the train to move, he engages the bell, which rings continuously, and sounds the air horns twice (indicating forward motion).

The throttle control has eight positions, plus an idle position. Each of the throttle positions is called a "notch." Notch 1 is the slowest speed, and notch 8 is the highest speed. To get the train moving, the engineer releases the brakes and puts the throttle into notch 1.

In this General Motors EMD 710 series engine, putting the throttle into notch 1 engages a set of contactors (giant electrical relays). These contactors hook the main generator to the traction motors. Each notch engages a different combination of contactors, producing a different voltage. Some combinations of contactors put certain parts of the generator winding into a series configuration that results in a higher voltage. Others put certain parts in parallel, resulting in a lower voltage. The traction motors produce more power at higher voltages.

As the contactors engage, the computerized engine controls adjust the fuel injectors to start producing more engine power.

The brake control varies the air pressure in the brake cylinders to apply pressure to the brake shoes. At the same time, it blends in the dynamic braking, using the motors to slow the train down as well.

The brake and throttle controls
The brake and throttle controls

The engineer also has a host of other controls and indicator lights.

Controls, indicators and the radio
Controls, indicators and the radio

A computerized readout displays data from sensors all over the locomotive. It can provide the engineer or mechanics with information that can help diagnose problems. For instance, if the pressure in the fuel lines is getting too high, this may mean that a fuel filter is clogged.

This computerized display can show the status of systems all over the locomotive.
This computerized display can show the status of systems all over the locomotive.

Now let's take a look inside the train.

Riding the Train

Inside a passenger car
Inside a passenger car

The accommodations inside a passenger train are quite plush. This train is the Piedmont, which runs daily from Raleigh to Charlotte, North Carolina. The seats on this train recline more than airline seats and have more leg room. They also have footrests.

The seats on this car can be turned around to face each other so four people can sit together.
The seats on this car can be turned around to face each other so four people can sit together.
The train also has a kitchen that serves mostly sandwiches and light snacks.
The train also has a kitchen that serves mostly sandwiches and light snacks.
For first-class passengers on this train, there is an observation car that has a sunroom upstairs and a bar.
For first-class passengers on this train, there is an observation car that has a sunroom upstairs and a bar.

Although taking the train might be slower than flying, it's definitely a lot more comfortable. There is plenty of room to walk around, and you can eat in a dining car or look at the view from the the top of the lounge car. Some trains even have private rooms for first-class passengers -- not a bad way to get from here to there.

For more information on diesel locomotives and related topics, check out the links on the next page.


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